Method of forming dental restorations

ABSTRACT

This invention relates to a novel method for preparing dental restorations and the product, thereof, in which a relatively hard alloy is utilized for casting a crown or inlay and a softer, more malleable alloy in particulate form is positioned in the mold prior to the casting operation in the marginal areas of the restoration to provide better adaptation to the gingival margins and/or to the tooth structure.

BACKGROUND OF THE INVENTION

This invention relates to a novel method for the production of dentalrestorations and products produced thereby which permits theadvantageous and economical use of gold in combination with otherconventional alloys such as silver-palladium, low gold contact alloysand the like. The greatly increased cost of gold as a preferred materialfor making dental restorations such as crowns, inlays and bridgework hasstimulated a greater degree of interest in the use of other less costlymaterials such as the alloys referred to. However, such alloys havecertain disadvantages since they do not permit suitable marginaladoption to the gums, in the case of crowns, or to the interiors ofcavities, in the case of inlays.

Casting gold alloys are classified by the American Dental Associationaccording to their hardness, as determined by their resistance toindentation, into four groups: soft, medium, hard and extra hard, TypesI, II, III, and IV respectively. Type I, for example, has a gold contentof about 80-90%, Type II, 73-83%, Type III, 71-80% Type IV, 62-72%.Silver, copper, palladium and zinc are present in varying proportions.

Dental casting alloys containing relatively low proportions of gold havebeen available for many years but are now attracting a high degree ofinterest due to the great increase in the price of gold. In general,these alloys are harder and more durable than high gold content alloys.A review of low gold content alloys is contained in a report by theAmerican Dental Association in JADA Vol. 100, February 1980, Pages237-240. Palladium - silver based alloys containing little or no gold,have been developed as substitutes for higher gold content alloys asreported in JADA Vol. 89, August 1874, Pages 383-385 and in literatureand patents referred to therein. Such alloys, as well as the low andmedium gold alloys referred to above, are highly suitable for use as theprimary component of the restorations to be used with the presentinvention. A serious problem inherent with the use of lower gold contentalloys, or such alloys as palladium-silver referred to, is the fact thattheir hardness precludes close marginal adaptation to the toothstructure and exposes more cement at the seal margins. Throughout thehistory of dentistry, gold or high gold alloys have been the metals ofchoice for crowns or inlays because of their marginal adaptivity, sincethe life of an inlay or crown depends upon good marginal adaptation andsmaller cement exposure.

SUMMARY OF THE INVENTION

Applicant has now discovered and developed a novel composite castingprocess which joins together in one operation within the casting molditself, two metals or alloys which permit the use of low or no goldcontent alloys as the major component of the restoration while providinga pure or higher gold content region located at the marginal area of theproduct to permit suitable adaptation to the tooth structure. This isaccomplished by forming grooves or marginal cut out areas in the waxpattern of the inlay or crown and incorporating in said grooves acomoposition composed essentially of particles of gold or a gold alloybound together with a small proportion of a binder to hold the particlestogether to permit manipulation by the dentist to comform to the desireddimensions of the pattern and fill in the cutout marginal areas. Thiscomposition of gold or high gold alloy particles preferably has a goldparticle content of 95% or more with the rest being a binder such as adental wax or other similar material such as petroleum wax or a gum suchas agar, gum arabic or similar natural or artifical gums or resins whichwill have suitable binding properties while still allowing the particlecomposition to be plastic enough to be worked in order to fill thegrooves referred to.

The gold alloy particles referred to may be of any desired gold contentas long as they form a film which is sufficiently malleable to beadapted to the margins of the tooth and are softer than the alloy of thecasting alloys having a gold content higher than 50% have been found tobe highly suitable.

In essence, applicant has invented a process which enables thepreparation of a restoration of the type described herein, in which themajor component, which is subject to the greatest stress and wear, isformed of a relatively hard and durable precious metal alloy with a lowgold content or one with no gold content and in which the marginal areaswhich require adaptation to the tooth structure and which represent onlya small portion of the total volume, are composed of a softer alloy ofhigher gold content to permit adaptation to the tooth or gums andpermits formation of a close marginal seal with minimal cement exposure.Since the softer margins are relatively thin and are blended with andsupported by the harder alloy forming the bulk of the restoration, theyare strongly supported by the harder base alloy in addition to beingsufficiently malleable to permit proper adaptation.

One such composition is preferably formed of finely divided gold powderor filings dispersed in wax, shaped in the form of a small diameter rodor narrow strip. Another such composition may be formed by mixing thegold or gold alloy particles with a dental utility wax with just enoughwax to make the mixture plastic to form a heavy paste.

In general terms the novel procedure referred to is utilized inconnection with conventional practice in the preparation ofrestorations. In so doing, an impression is prepared from the patient'smouth. At the laboratory a "stone" or plaster cast is prepared and aworking model of the tooth is produced. The male casting, whichrepresents the patient's tooth, after preparation to define the cavityor contour to which the inlay or crown is to be supplied, is called the"die". In conventional practice, a wax inlay pattern or wax crown isformed on the "die", a wax sprue is attached and the wax patterern ispositioned in a casting ring. An investment material is then used tofill the casting ring and after it sets, is heated to remove the wax.The investment casting then has cavity corresponding to the removed waxand sprue and is then positioned in association with a centrifugalcasting machine. A pellet of gold or alloy which is to be used in therestoration is melted and cast into the investment cavity by centrifugalforce.

In carrying out the present invention, grooves or marginal cut-out areasare formed in the wax inlay pattern, preferably 1 to 1.5 mm in width anddepth, or in the case of a crown, gingival margins are cut out to asimilar width and depth and the composite formulation containingparticulate gold or gold alloy dispersed therein is used to fill in thecut out areas. In one convenient procedure, the gold-wax composition isfirst formed into rods having a diameter conforming to the dimensions ofthe cut out margins and such rods are inserted or laid into saidmargins, as more fully described below. The wax-gold composition isformed of a wax such as a soft dental utility wax also known as boxingor carding wax into which 95 to 98% by volume of dust or filings orsimilar particles of gold or a high gold alloy having a particular sizeranging from that of dust of a few microns to larger particles up to say0.25 millemeters in diameter. A preferred particle size has been foundto be about 0.01 millemeters. This is done by softening or melting thewax, then adding the gold particles while stirring to disperse the goldand at the same time, allowing the wax to harden. To form the rods, thecomposition may be extruded to the proper diameter and convenient lengthfor handling. Instead of rods, the wax-gold composition may be formedinto strips of rectangular or oval cross sections. These rods may thenbe packaged in lengths ranging from 2-6 inches.

As an alernative to the use of particle composition in the form of rods,the gold or gold alloy particles, after mixing with the wax, or otherselected binder, is formed into a heavy paste and the paste material isapplied to the grooves in the inlay or crown patterns by means of smallspatula and shaped to conform to the final shape of the pattern. Thismaterial may be packaged in jars or collapsible tubes.

In practice, the gold particle composition, if in the form of a rod orcylinder, is pressed into the cut-out groove with finger pressure andthen pressed more firmly into the groove with a flat blade waxinsturment. If a paste formulation is used, it can be applied to thegroove with a suitable tool. After the particle composition is in place,it is luted to the body of the wax pattern with the tool and if the waxbinder is used, the tool may be heated. In any case, the contour of thepattern is carefully restored in the filling in the cut out area. Theresulting composite wax pattern carrying the wax-gold composition has asprue applied and is removed from the "die" and invested into a ring forcasting in the conventional manner. When the wax is removed from thecasting mold ring, prior to the final casting with the desired alloy,the gold or gold alloy from the wax marginal insert is deposited alongthe desired marginal areas and retains its form and position within orupon the final "die", and during the alloy casting operation, is meltedand either is uniformaly bonded in a thin film along the marginal areasof the cast alloy or is surface alloyed depending on the composition ofthe casting material. In either case, the critical marginal area is ofgold or of substantial gold content so that it may be readily adapted tothe tooth surface and/or the gingival areas as the case may be. Thesprue referred to above may be of wax or of plastic as in conventionalpractice.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a view in perspective of a stone die conforming to a toothhaving a prepared cavity.

FIG. 2 is a view similar to FIG. 1 in which a wax inlay pattern has beenformed.

FIG. 3 is a view showing the wax pattern of FIG. 2 with wax margins cutout.

FIG. 4 is a view of a wax rod having gold particles dispersed therein.

FIG. 5 is a view showing the cut-out margins of FIG. 3 with the rod ofFIG. 4 in place.

FIG. 6 is a view in perspective showing a wax sprue attached to the waxpattern.

FIG. 7 is a view showing the wax pattern and sprue assembly of FIG. 6positioned in an investing ring.

FIG. 8 is a view showing the inlay with sprue attached to preparatoryfinal polishing.

FIG. 9 is a view in perspective of a completed inlay in a tooth.

FIG. 10 is a view in partial vertical cross section of the inlay of FIG.9.

FIG. 11 is a view in perspective of a die conforming to a molar, withpreparation for a crown.

FIG. 12 is a view showing the die of FIG. 11 provided with a wax crownconforming to the desired molar configuration.

FIG. 13 is a view in perspective showing the wax-gold compositionincorporated along the gingival area.

FIG. 14 shows a finished molar alloy crown in place on a tooth with softadaptable gold margins.

FIG. 15 is a view showing the crown of FIG. 14 in vertical crosssection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in detail, as shown in FIG. 1, a stone die 20is prepared from the impression of the tooth as described above, in thecase of a cavity to be corrected by means of an inlay, the cavity afterpreparation by the dentist is shown at 21. In FIG. 2, inlay wax 22 isapplied to the cavity in the proper manner. In FIG. 3, the margins ofthe wax outlining the cavity are cut out or recessed by a hand tool or asmall burred outlining tool operated by an electric hand drill to form acontinuous groove 23 to accommodate the insertion of a strip or rod ofthe wax-gold particle composition 24a illustrated in FIG. 4. Thedimensions of the groove, and of the wax-gold composition may vary butpreferably are about 1 to 1.5 mm. in width and depth. The wax-goldcomposition is pressed into the groove and the surface smoothed out toproperly conform to the die as shown in FIG. 5. A sprue 25 is thenattached to wax 24 in conventional manner as shown in FIG. 6. The waxinlay pattern, with the wax-gold marginal insert and the attached sprue,are then removed from the die and positioned within an investmentcasting ring 26 as shown in FIG. 7. The ring is then filled withinvestment material surrounding the pattern and after setting of thematerial, it is heated and the wax melted or burnt at a suitabletemperature, say 1200° F., leaving a cavity corresponding to the cavityto be replaced with the inlay. At the same time, the gold particlescontained in the marginal wax-gold insert remain in position in thecavity.

The mold is now ready for casting and this is carried out inconventional manner as by centrifugal casting of a molten pellet of thealloy to be used. One suitable alloy is a conventional silver-palladiumalloy known to the dental professional as "Alborium", containing 15% Av,25% Pd and 45% Ag, the casting temperature of which is 2050°-2150° F.The mold itself, remains heated for example at about 1000° F. while thecasting alloy is introduced in its molten state.

After casting and cooling of the mold, the inlay 27 with a metal sprue25 attached, is mounted on the die for removal of the sprue and finalpolishing as shown in FIG. 8. As shown, the casting is composed of thealloy 27 with the marginal areas which would be in contact with thesurface of the tooth composed of gold or gold alloy 27a bonded to and/oralloyed with the material of the alloy. When the inlay is installed inthe tooth cavity, the higher gold content margins are adaptable and canbe fitted and polished to provide the necessary smooth and tightlysealed surface.

FIGS. 9 and 10 illustrate the inlay 27 with marginal softer alloy 27ashown as a distinct region but in fact, the marginal segment isintimately bonded to and alloyed with the alloy of the inlay 27.

FIGS. 11, 12, 13, 14 and 15 illustrate the invention as applied to amolar crown. In FIG. 11, stone die 28 is shown with area 29 shown in thedotted lines conforming to the surface of the tooth prior to preparationand 30 showing the prepared surface. In FIG. 12, wax crown 31 is appliedto the die and FIG. 13 shows the wax crown 31 with the marginal orgingival area shown with the gold-wax composition 32 inserted aftercutting out a groove having the necessary dimensions, which as discussedwith respect to the inlay, are preferably 1-1.5 mm. wide and deep

FIG. 14 shows the finished Crown 33 with the marginal soft alloy area 34while FIG. 15 shows the crown 33 with the soft marginal portion 34 invertical cross section. Although the marginal area is shown with a lineof demarcation, this is for purposes of illustration, since in practice,the soft and hard components are alloyed together at the junction and asharp demarcation does not occur.

After investment, removal of the wax, and casting with the desiredalloy, i.e. silver-palladium, the finished crown is cemented to thetooth in conventional manner. The gingival margins being composed ofhigher gold content alloyed to the silver-palladium permits properadaptation to the tooth and gum.

I claim:
 1. A method of forming dental restorations which comprisesincorporating within a mold cavity conforming to the shape of thedesired restoration finely divided particles of gold or relatively softgold alloy only along an area corresponding to the exposed marginal areaof the restoration contiguous to the tooth or gum, and thereaftercasting within said cavity a relatively harder dental alloy, saidparticles and said harder alloy becoming bonded together to form acomposite structure in which softer marginal areas are formed to permitimproved adaptation of the restoration to the tooth and/or gum.
 2. Amethod according to claim 1 wherein the relatively hard alloy is analloy selected from the class consisting of alloys comprising gold,metals of the platinum group, silver, and mixtures thereof, and thesofter alloy comprises gold or an alloy thereof.
 3. A method of formingdental restorations which comprises forming a wax pattern correspondingto the restoration to be applied to a tooth, forming a groove aroundedges of the pattern along the margins thereof, filling said groove withparticles of gold or gold alloy, surrounding the resulting wax patternwith investment material, heating to remove the wax and thereby forminga mold cavity with gold particles deposited therein along areascorresponding to the margins of the desired restoration, and castingwithin said cavity a dental alloy of desired hardness and at the sametime melting said gold particles whereby said alloy becomes bonded tosaid gold to form a soft gold containing marginal area.